The present invention relates to organic electroluminescent devices (“OELDs”). In particular, the present invention relates to such devices having a plurality of electroluminescent elements arranged in a stack.
Electroluminescent (“EL”) devices, which convert electrical energy to electromagnetic energy, may be classified as either organic or inorganic and are well known in graphic display and imaging art. EL devices have been produced in different shapes for many applications. Inorganic EL devices comprising inorganic semiconducting materials have gained acceptance in many applications in recent years. However, they typically suffer from a required high activation voltage and low brightness. On the other hand, organic EL devices (“OELDs”), which have been developed more recently, offer the benefits of lower activation voltage and higher brightness in addition to simple manufacture, and, thus, the promise of more widespread applications.
An OELD is typically a thin film structure formed on a substrate such as glass or transparent plastic. A light-emitting layer of an organic EL material and optional adjacent organic semiconductor layers are sandwiched between a cathode and an anode. The organic semiconductor layers may be either hole (positive charge)-injecting or electron (negative charge)-injecting layers and also comprise organic materials. The material for the light-emitting layer may be selected from many organic EL materials that emit light having different wavelengths. The light-emitting organic layer may itself consist of multiple sublayers, each comprising a different organic EL material. State-of-the-art organic EL materials can emit electromagnetic (“EM”) radiation having narrow ranges of wavelengths in the visible spectrum. Unless specifically stated, the terms “EM radiation” and “light” are used interchangeably in this disclosure to mean generally radiation having wavelengths in the range from ultraviolet (“UV”) to mid-infrared (“mid-IR”) or, in other words, wavelengths in the range from about 300 nm to about 10 micrometers. Although OELDs now require relatively low activation voltages, the continuous activation of these devices still contributes to limit their longevity.
Therefore, their still is a need to provide OELDs having longer life. Moreover, it is very desirable to provide light sources comprising OELDs that have longer life but do not suffer from diminished illuminating capacity.